Add ability to globally fit peaks to calculate shared parameters

[MialLewis]

We want to add the ability to globally fit peaks to calculate shared parameters.

For this stage of the project, the following sub-issues need to be resolved:

• Add ability to globally fit peaks
• Add calculation of E1 value with globally fitted peaks.
• Add calculation of L1 value with globally fitted peaks
• Add ability to specify whether peaks should be individually fit, globally fit, or both.
• Add ability to manually specify invalid detectors, instead of using the function.
• Refactor global fitting capability to reduce code reuse where possibe.
• Increase unit test coverage of new global fitting capability.

[MialLewis]

I'm working on Add ability to manually specify invalid detectors, instead of using the function.

[jess-farmer]

I've been working on Add ability to specify whether peaks should be individually fit, globally fit, or both. - I'll open a PR with these changes later today

[MialLewis]

I've been working on Add ability to specify whether peaks should be individually fit, globally fit, or both. - I'll open a PR with these changes later today

To warn, there's quite a big PR #61 in bound that will separate the calibration script file so that the two different classes are held in different files.

I suspect this PR will be merged before yours, so it may be worth rebasing on the associated branch sooner rather than later.